Enthalpy in Changes of State

Enthalpy of Solutions & Changes of State

Multiple Choice

The SI unit of heat and energy is the __________.

• calorie

• heat

• joule

• watt

Multiple Choice

How do you calculate the Enthalpy of a reaction?

• ΔH = ΔH_products - ΔH_reactants

• ΔT = q / mC

• ΔG = ΔH -TΔS

• E = mc²

Multiple Choice

ΔH value in an endothermic reaction is a positive number.

• True

• False

Multiple Choice

CaO(s) + H₂O(l) ⟶ Ca(OH)₂(s)     ΔH = −65.2 kJ

Is the above reaction exothermic or endothermic?

• Exothermic

• Endothermic

Multiple Choice

Endothermic reactions feel

• warm

• cold

Multiple Choice

How much heat does an aluminum block absorb if 10.00 grams ae heated from 25.0^oC to 50.0^oC? the specific heat of aluminum is 0.900J/g^oC

• 450

• -450

• 225

• -225

Enthalpy of Solution

• When a solution is formed, heat is either released or absorbed

• Molar Enthalpy of Solution: the enthalpy caused by the dissolution of one mole of substance

• Water molecules are polar, and effective of breaking the bonds of both molecular and ionic structures

  • The breaking of those bonds is an endothermic process, because it needs to overcome the bonds already in place

  • The enthalpy of solution is proportional by moles

Experience Chemistry | Lesson 8.3

Enthalpy of Solution Sample Problem 1

• When one mole of NaOH_(s) is dissolved in water, 44.5 kJ of heat are released. How much heat is released when 2.50 moles of NaOH is dissolved in water?

  • Step 1: Identify what you know

    • Δ​H_sol= -44.5 kJ/mol

      • Heat is released in an exothermic reaction, so ΔH is negative

    • Amount of NaOH dissolved= 2.50 mol

Experience Chemistry | Lesson 8.3

Enthalpy of Solution Sample Problem 1

• When one mole of NaOH_(s) is dissolved in water, 44.5 kJ of heat are released. How much heat is released when 2.50 moles of NaOH is dissolved in water?

  • Step 1: Identify what you know

    • Δ​H_sol= -44.5 kJ/mol

    • Amount of NaOH dissolved= 2.50 mol

  • Step 2: Write the known ΔH as a conversion factor

Experience Chemistry | Lesson 8.3

Enthalpy of Solution Sample Problem

• When one mole of NaOH_(s) is dissolved in water, 44.5 kJ of heat is released. How much heat is released when 2.50 moles of NaOH ar dissolved in water?

  • Step 1: Identify what you know

    • Δ​H_sol= -44.5 kJ/mol

    • Amount of NaOH dissolved= 2.50 mol

  • Step 2: Write the known ΔH as a conversion factor

  • Step 3: Use your number of moles and the conversion factor

Experience Chemistry | Lesson 8.3

Enthalpy of Solution Sample Problem 2

Experience Chemistry | Lesson 8.3

Enthalpy of Solution Sample Problem 2

Experience Chemistry | Lesson 8.3

Enthalpy in Changes of State

Experience Chemistry | Lesson 8.3

• ​As materials are heated, there temperature will change up to a point

  • At a certain point, temperature will stop increasing because the material undergoes a phase change

    • Can occur when adding or removing heat

• Phase changes do not form or break bonds within molecules, but increase the kinetic energy of each individual molecule​

Enthalpy of Fusion & Solidification

Experience Chemistry | Lesson 8.3

• Molar enthalpy of fusion ΔH_fus: the heat absorbed by one mole of a solid substance as it melts to a liquid at a constant temperature

• Molar Enthalpy of Solidification ΔH_solid:​ the heat lost when one mole of a liquid substance solidifies at constant temperature

• The heat absorbed by a melting solid is the same as the the heat released by the liquid solidifying​

  • ΔH_fus​=ΔH_solid

Enthalpy of Vaporization & Condensation

Experience Chemistry | Lesson 8.3

• Molar enthalpy of Vaporization ΔH_vap: the heat absorbed by one mole of a solid substance as it vaporizes at constant temperature

• Molar Enthalpy of Condensation ΔH_cond:​ the heat lost by one mole of a gas substance as it condenses at its normal boiling point

• The heat absorbed by a vaporizing liquid is the same as the the heat released by a condensing gas

  • ΔH_fus​=ΔH_solid

Structure and Changes of State

Experience Chemistry | Lesson 8.3

• The molecules of a substance do not change during a change of state, and no chemical bonds are broken

  • The only way that changes is how the molecules arrange themselves, which is related to each molecules molecular force​

    • The Strengths of intermolecular forces and the sizes of particles can be used to predict the relative amount of energy required to change the phase of a substance

Experience Chemistry | Lesson 8.3

Evaporation

Experience Chemistry | Lesson 8.3

• Vaporization can occur before a liquid hits its boiling point

  • While most of the particles do not have enough energy to overcome the intermolecular state and vaporize, some individual kinetic energies at the surface of the liquid are able to break free

Multiple Choice

True or false: Every time you add energy into a system, the temperature will increase

• true

• false

Match

Match the following Enthalpies to their definitions

• Molar Enthalpy of Solution

• Molar enthalpy of fusion

• Molar Enthalpy of Solidification ΔH_solid

• Molar enthalpy of Vaporization ΔH_vap

• Molar Enthalpy of Condensation ΔH_cond

the enthalpy caused by the dissolution of one mole of substance

the heat absorbed by one mole of a solid as it melts to a liquid at a constant temperature

the heat lost when one mole of a liquid substance solidifies at constant temperature

the heat absorbed by one mole of a solid substance as it vaporizes at constant temperature

the heat lost by one mole of a gas substance as it condenses at its normal boiling point

• the enthalpy caused by the dissolution of one mole of substance

• the heat absorbed by one mole of a solid as it melts to a liquid at a constant temperature

• the heat lost when one mole of a liquid substance solidifies at constant temperature

• the heat absorbed by one mole of a solid substance as it vaporizes at constant temperature

• the heat lost by one mole of a gas substance as it condenses at its normal boiling point

Multiple Choice

True or false: A liquid substance can vaporize without hitting the boiling point

• true

• false